1. Field of the Invention
This invention relates to a process for preparing thermoplastic vulcanizates (TPV) using granular gas phase ethylene-propylene-diene terpolymer.
2. Description of the Related Art
Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic. These rubber particles are crosslinked to promote elasticity. Thermoplastic vulcanizates are conventionally produced by dynamic vulcanization, which is a process whereby a rubber is cured or vulcanized within a blend with at least one non-vulcanizing polymer while the polymers are undergoing mixing or masticating at some elevated temperature, above the melt temperature of the non-vulcanizing polymer. See, for example U.S. Pat. Nos. 4,130,535, 4,311,628, 4,594,390 and 6,147,160.
Granular gas phase ethylene-propylene-diene terpolymer (EDPM) thermoplastic vulcanizates are known, see EP 0 775 718, and “NORDEL® MG—The Game Changer . . . for TPV,” TPE 2003, Brussels, Belgium—16-17 Sep. 2003. As noted in these descriptions, granular gas phase EPDM rubber (GPR) presents an advantage over solution or slurry polymerized EPDM rubber in that the GPR is in a granular form upon exit from the gas phase reactor such that it is in a preferred form for introduction to plastic compounding equipment. Carbon black, or other filler, is added in the gas phase reactor to prevent rubber particle agglomeration and sticking to reactor walls, valves, piping, and the like. This forms the particulate GPR. The solution or slurry polymerized EPDM rubber, upon exit from its respective reactor must have excess solvent or reaction medium removed, after which the rubber is in bulk form. It is then often shipped to a compounder in bale form. The bale must thereafter be pre-compounded into particulate form, such as crumb, often with addition of some filler or powder, etc., as a partitioning agent to inhibit the inherent tendency of the rubber crumb to agglomerate before it is suitable for feeding into the compounding equipment. This is typically true in the preparation of conventional TPV compositions.
Conventionally produced TPV compositions also often include carbon black as a filler, where a black color is desired or is acceptable. Although carbon black is typically added to the composition prior to dynamic vulcanization, conventional wisdom suggests that the carbon black becomes primarily incorporated into the plastic matrix of the TPV. As a result, the advantages associated with carbon black, such as the UV stability, are not believed to be fully realized in the rubber phase. GPR avoids this, and includes the carbon black principally in the rubber phase. See WO 03/059963. This document as well notes that high molecular weight rubber, very high Mooney viscosities, can be prepared by this process. Such high molecular weight rubber can be advantageously used in TPV compositions and are thus highly desirable.
Very high molecular weight EPDM rubber can be very hard to manufacture and process. As is well known in the art of EPDM rubber, the ease of processing goes down significantly with significant increase in molecular weight. Often such EPDM is thus oil extended. This creates a problem where the choice of processing oil is taken from the compounder and the best choice for a given TPV product may be compromised. But with GPR, processing oil is added in the TPV reactive melt processing and may be selected and appropriate in view of the choice of thermoplastic resin and other additives. A proposed preparation of TPV compositions from GPR, including adding oil in TPV processing using the high Mooney embodiments, is described in the TPE Paper, supra.
Because the number of uses of thermoplastic vulcanizates is increasing, the performance demands that are placed on these materials is more demanding, and the manufacturing efficiency of the materials is continually pursued, there exists a need to overcome some of the shortcomings associated with the prior methods of manufacture.